VANCOUVER — Radioactivity from Japan’s crippled nuclear reactors has turned up off the British Columbia coast and the level will likely peak in waters off North America in the next year or two, according to a Canadian-led team that’s intercepted the nuclear plume.

The radioactivity “does not represent a threat to human health or the environment,” but is detectable off Canada’s west coast and the level is climbing, a team led by oceanographer John Smith at Fisheries and Ocean Canada (also known as the Department of Fisheries and Oceans) reported Monday in the Proceedings of the National Academy of Sciences.

The team’s seawater measurements reveal Fukushima radioactivity first showed up 1,500 kilometres west of British Columbia in June 2012, more than a year after the Japanese nuclear accident.

By June 2013, the “Fukushima signal” had spread onto the Canadian continental shelf off the B.C. coast, and by February 2014, it was detectable “throughout the upper 150 metres of the water column,” says the report, showing how the Pacific currents are carrying the radioactive plume slowly across the ocean. It says the Fukushima’s radioactive signal off the B.C. coast is now double the “background” radiation in the ocean from atmospheric nuclear bomb testing.

The scientists predict the Fukushima radioactivity off North America will continue to increase before peaking in 2015-16 at levels comparable to those seen in the 1980s as a result of nuclear testing. Then levels are expected to decline and, by 2021, should return to levels seen before that Fukushima accident — considered one of the most serious nuclear reactor accidents.

A huge earthquake off the coast of Japan in March 2011 triggered a tsunami that flooded the Fukushima Dai-ichi nuclear power plants. Loss of backup power led to overheating, nuclear meltdowns and evacuation of the Fukushima site. Land and farms around the nuclear plants were severely contaminated and a large radioactive discharge washed into the Pacific.

Smith’s work is part of an ocean monitoring program set up to trace the plume. Canadian Coast Guard ships travelling up to 1,500 kilometres off the B.C. coast and into the Beaufort Sea are collecting the seawater from depths of up to 1,000 metres and the scientists are testing it for the radioactive isotopes Cesium-137 and Cesium-134.

Cesium-137 is the bigger concern, as it lingers in the environment for decades. Cesium-134 decays much faster but is “an unequivocal fingerprint indicator of contamination from Fukushima,” the scientists say. This is because the Japanese reactor accident is the only large contributor of the compound into the Pacific Ocean other than fallout from nuclear bomb testing which peaked in the 1960s and has been dropping since.

While the Cesium-134 from the accident will disappear within a few years, Cesium-137 can linger for years.

Thus, the scientists predict the Cesium-137 levels off the North American coast will not return to the levels seen before the Fukushima accident until 2021.

Smith, who leads the radioactivity section at DFO’s Bedford Institute of Oceanography,was unavailable for an interview before deadline. But the report says it is the first systematic study “of the arrival of the Fukushima radioactivity signal in continental waters off North America.” It says the findings “are critical” to an understanding of the circulation of the radioactivity.

But the scientists say it poses no harm.

The level of Cesium-137 in the water is far below levels seen in the 1960s and 1970s from nuclear weapons testing and “well below Canadian guidelines for drinking water quality,” they say.

And the dose of radiation expected by consuming bluefin tuna exposed to radioactivity from Fukushima accident is “comparable to the dose commonly received from naturally occurring radionuclides in many other food items, and only a small fraction of doses from other background sources.”

twitter.com/margaretmunro

NO NEED TO FREAK OUT

The background level for Cesium-137 in the Pacific Ocean is about one becquerel (Bq) — the decay of one Cesium-137 nucleus per second — per cubic metre of seawater. Fukushima has increased the radiation level off the B.C. coast to about 2 Bq and the level is expected to peak about 3 to 5 Bq per cubic metre of water by 2015-16. Canada’s drinking-water standard for Cesium-137 is 10,000 becquerels (10 kBq) per cubic metre.

Margaret's work covering science - and science controversies - has taken her to the Arctic to see the effects of global warming, to Cape Canaveral for space launches and into Ottawa's paper labyrinth ... read moreto reveal how the Canadian government has been muzzling scientists.View author's profile